This nationwide census conducted in psychiatric institutions and outpatient clinics showed that substance use occurred more frequently among in-patients than outpatients, and among patients with more severe mental illness in both treatment settings. Among in-patients, the most frequently used substances were cannabis and sedatives, whereas alcohol was more prevalent than use of any illegal substances among outpatients. In both samples, substance use was more prevalent in young males and the most socially deprived.
The higher prevalence of both SUD diagnosis and substance use found among in-patients aligns with previous studies [7, 14], and suggests that in in-patient treatment, there is a different patient case mix with relatively higher rates of severe mental illness, such as schizophrenia and major depression, which are typically associated with higher rates of co-occurring SUD [3, 7, 10].
A number of studies have documented that substance use and SUD are under-reported and underdiagnosed in psychiatric treatment settings [24–26]. Moreover, a recent Norwegian Board of Health study highlighted the existing deficiencies in specialized mental health services regarding the investigation and examination of patients with possible concurrent SUD . The accuracy of reported substance use, such as in the present study, may depend on several factors, including whether substance use issues were part of the routine clinical assessment, and the patients’ responses when asked about substance use. Such conditions may vary between treatment settings . It is possible that the intensive treatment and close monitoring of patients in in-patient treatment allowed clinicians to make a more thorough evaluation of the patients’ substance use behaviour, resulting in higher identification rates than in outpatient treatment settings. The under-identification of substance use may also be related to insufficient knowledge of substance abuse issues among clinicians  and/or the lower status of working with substance users .
Methodological and sample differences complicate the comparison of results between studies. Of the few studies conducted in general psychiatric outpatient and in-patient treatment settings, Rush and Koegl’s study , conducted in in-patient and outpatient mental health treatment facilities in Ontario, is most comparable with the present work. They reported a prevalence of SUD (based on diagnosis) among in-patients of 15.6%, which is similar to the point prevalence of recorded SUD diagnosis in the current in-patient sample (16.4%). However, the current SUD rate of 5.6% based on recorded diagnosis found in the outpatient sample is relatively low in comparison with the 11% SUD prevalence among outpatients reported by Rush and Koegl .
When comparing the prevalence of substance use in psychiatric patients across countries, one should take into account possible between-country differences in the organization and delivery of health services. For example, community addiction services have a more prominent role in Norway and other European countries than in the USA . Moreover, in Norway, community-integrated mobile teams offer dual treatment for individuals with mental illness and substance abuse.
Consistent with results from other studies of psychiatric patients, the present results showed that alcohol, cannabis and sedatives were the most commonly used substances [3, 11, 12]. Also in line with previous reports [7, 10], use of illegal substances, particularly cannabis, occurred more frequently among in-patients than outpatients, and, as hypothesized, among patients with the most severe mental disorders (e.g., schizophrenia and other psychoses). Although the association between use of illicit drugs and severe mental illness is frequently recognized in the literature, the basis for the link is still being discussed [30–32]. For example, research concerning the association between schizophrenia and SUD has suggested a common genetic risk , or that patients with a genetic risk for schizophrenia are more vulnerable to substance use .
As hypothesized, and consistent with previous clinical studies, the current results showed that substance use occurred twice as often among men than among women [e.g., 7, 9]. The prevalence of substance use was particularly high among in-patient young males, aged 18–29 years. This finding may reflect the high prevalence of substance-induced mental disorders among in-patients, which is associated with symptom onset at a younger age [7, 35]. Our results also confirmed the socio-demographic differences in prevalence of substance use reported elsewhere .
After adjusting for demographic and socio-demographic variables in multivariate analysis, the differences in prevalence rates of substance use between the psychiatric diagnostic categories among in-patients were reduced, indicating that substance use was associated with greater levels of social and economic disadvantages, as reported elsewhere . The present results showed that independent of demographic and socio-demographic factors, in-patients with anxiety disorders, who constituted a relatively small subsample of those in in-patient treatment (n = 62), had the highest prevalence of substance use, particularly alcohol and sedatives; this suggests that this subsample constituted a group of patients who used substances to cope with severe anxiety symptoms [36, 37].
A Canadian study found that socio-demographic characteristics did not explain differences in substance use prevalence among in-patients, while the opposite was found for outpatients . Our results imply that socio-demographic characteristics explain differences in prevalence of substance use among both in-patients and outpatients. Furthermore, Rush and Koegl  found that among outpatients, socio-demographic variables such as young age, being male, single, and having a low level of education were associated with prevalence of substance use. Our results indicate that outpatients in the youngest age group (18–23 years old) were at lower risk of substance use than patients older than 24 years. However, we also found that less of the difference in prevalence of substance use between psychiatric disorders was explained by socio-demographic variables among outpatients than in-patients, reflecting a relatively homogeneous outpatient population. Differences in access to health services between countries, and subsequent differences in patient composition in specialty mental health services might explain the non-congruent findings.